The present invention relates generally to the field of signal processing and, more specifically, to the problem of the generation of the phase-conjugate or time-reversal of an input signal. Still more specifically, the present invention relates to the generation of this phase-conjugate or time-reversed signal through the use of surface acoustic wave (SAW) devices.
Microwave traveling wave tubes are known to be capable of providing phase-conjugate reflections with identical features, i.e. described by the same equations. Microwave frequency surface wave devices have several advantages over their volume wave counterparts. Surface waves require only one optically polished surface whereas volume waves require two surfaces which must be parallel to optical tolerances. The fabrication techniques for surface wave transducers are the same as those used for integrated circuits so that SAW devices can be fabricated on a substrate together with transistor components. Also, SAW devices are inherently small, lightweight and rugged and are therefore particularly adapted to airborne and aero space applications. Additionally, SAW devices generally are not affected significantly by Joule heating problems.
No specific SAW devices have been reported utilizing the degenerate mixing of an input probe wave with two steady-state counterpropagated pump waves. However, SAW devices have been reported which achieve parametric amplification and four-wave interaction.
The earlier devices using piezoelectric semiconductor construction in which parametric amplification has been observed consist of a metal-insulator-semiconductor (MIS) structure, Al/Zn/SiO.sub.2 /Si. Across the device was an externally applied pump voltage at twice the frequency of the input acoustic wave which modulated the space charge layer in the silicon and hence the propagation properties of the acoustic wave which was launched from an interdigital transducer on the zinc oxide. Similar piezoelectric semiconductor devices using LiNbO.sub.3 and Si have been constructed and tested in acoustoelectric experiments on memory devices and FM demodulators.